In a digital mobile communications system which adopts the TDD (Time Division Duplex) method, a base station transmits a downlink signal to a wireless radio station and the radio station transmits an uplink signal to the base station by alternatively using the entire bandwidth of a transceiver (transmitter-receiver) for a portion of the time corresponding to one of periodically recurring time slots.
FIG. 5A illustrates an exemplary communications system adopting the TDD method. The communications system includes a base station 21 and a mobile station 22.
FIG. 5B depicts communication slots used in the TDD method. In FIG. 5B, the horizontal axis represents time and uplink slots and downlink slots are arranged in the communication slots alternatively.
As mentioned above, since the communication is performed in both the uplink and the downlink by using an identical frequency band alternatively, a communication signal from the base station 21 to the mobile station 22 and that from the mobile station 22 to the base station 21 can be treated, according to the principle of reversibility, as passing through transmission channels bearing an identical fading characteristic. Based on the principle of reversibility, a transmission channel condition in a communication slot to be used in sending a signal from a first transceiver to a second transceiver can be estimated by using C/N (Carrier to Noise Ratio) or a delay spread of the transmission channel obtained from a signal transmitted from the second transceiver to the first transceiver during a previous communication slot.
Based on the estimation result of transmission channel condition, an optimal bandpas modulation method is determined so that a transceiver can meet a predetermined error rate and obtain a maximum transmission rate. The determined modulation method is used in transmitting a signal to its counterpart communications transceiver. A modulation method can be converted to another by changing a symbol rate or an M-ary modulation value (modulation level).
If the basic symbol rate is T, it can be changed to T/2, T/4, T/8, and so on. The change of M-ary modulation value can be exemplified by the conversion among, e.g., BPSK(Binary Phase Shift Keying), QPSK(Quadrature Phase Shift Keying), 16QAM(Quadrature Amplitude Modulation), 64QAM, 256QAM, and so on. The M-ary modulation value of BPSK is equivalent to 2, and that of QPSK is equivalent to 4.
When C/N is small or the delay spread is large, the transmission channel condition is considered to be poor so that the symbol rate and the M-ary modulation value can be made small. On the contrary, when C/N is large or the delay spread is small, the transmission channel condition is considered to be good so that the symbol rate and the M-ary modulation value can be made large.
Further, the type and/or the number of the symbol rates and the M-ary modulation values can be varied adaptively according to the type of the communications system being employed.
As described above, the transmission efficiency can be improved by using an adaptive modulation method capable of optimizing a modulation method based on the quality of the communications channel estimated from a received signal.
One of such prior art transceivers for use in a communications system adopting an adaptive modulation method is described by Matsuoka, Uhe, Sambei and Morinaga in “Analysis of transmission characteristics of adaptive modulation method which converts symbol rates and modulation values”, IEICE Technical Report, RCS94-64, 1994-09.
Referring to FIG. 6, there is illustrated a transceiver adopting such an adaptive modulation method described above, which includes an antenna 31, a reception circuit 32, a transmission channel estimation circuit 33, a modulation level control circuit 34, a frame configuration circuit 35, an orthogonal modulation circuit 36 and a transmission power amplification circuit 37.
FIG. 7 shows a frame structure used in the data communication by the transceiver shown in FIG. 6. As shown in FIG. 7, the frame includes a preamble 41 used in estimating a transmission channel condition, modulation method information 42 representing a modulation method used in transmission, and an information symbol 43 carrying the transmission data modulated by using the modulation method indicated by the modulation method information 42.
The transceiver shown in FIG. 6 receives a signal through the antenna 31. And then at the reception circuit 32, orthogonal detection of the received signal is carried out and then the modulation method to be used in modulating the received signal is determined based on the modulation method information 42 and finally the received signal is decoded to obtain received data. The transmission channel estimation circuit 33 detects C/N or the delay spread of the transmission channel and estimates transmission channel condition for a next data transmission. The estimation process at the transmission channel estimation circuit 33 is executed by using reception level information such as reception baseband signal and RSSI (Received Signal Strength Indicator) outputted from the reception circuit 32. Based on the estimation result, the modulation level control circuit 34 controls the modulation value to be used in a next data transmission.
When the transceiver transmits a signal, the frame configuration circuit 35 maps transmission data to symbols of the modulation method corresponding to a modulation value designated by the modulation level control circuit 34 and adds additional information, such as the modulation method information 42 and the preamble 41, to the mapped data, i.e., the information symbols 43, so that the frame shown in FIG. 7 is configured. The orthogonal modulation circuit 36 performs orthogonal modulation of the frame outputted from the frame configuration circuit 35. The modulated signal is amplified by the transmission power amplification circuit 37 and transmitted into the air through the antenna 31.
Referring to FIG. 8, characteristics of communication by modulation method will be described.
FIG. 8 shows examples of symbols in case of using BPSK, QPSK(4QAM) and 16QAM, the modulation values thereof being 2, 4 and 16, respectively.
As shown in FIG. 8, the larger the modulation value, the larger the amount of information transmitted and the poorer the reliability of communication. On the contrary, as the modulation value becomes smaller, the amount of information transmitted becomes reduced but with the better communication reliability.
In the adaptive modulation method, a data transmission is performed by using a modulation method of high reliability when the condition of transmission channel is considered to be poor. On the other side, when the condition of transmission channel is considered to be satisfactory, the modulation level control circuit 34 controls the modulation method used in transmission so that a large volume of information can be transmitted. Further, when the condition of the transmission channel is estimated to be worst, dummy data can be transmitted instead of real transmission data.
In this way, the quality of information transmitted, which can be estimated, e.g., by a bit error rate, can be improved.
Another exemplary conventional communications method is described in Japanese Patent Laid-Open Publication No.1998-247955. The digital mobile wireless communications method disclosed therein employs a multi-carrier transmission method utilizing a plurality of sub-carriers, which classifies the transmission channel condition of each sub-carrier into a plurality of levels, chooses a modulation scheme among 64QAM, 16QAM, QPSK and DUMMY by comparing reception condition with a threshold value, and transmits more (less) information through a sub-carrier which has better (worse) transmission channel condition.
Recently, a next generation broadband subscriber wireless access system (FWA system: Fixed Wireless Access system) has been developed and it is under consideration to apply the adaptive modulation method as described above in such an FWA system.
In the FWA system, an antenna of a wireless base station is installed on the rooftop of a building in order to transmit a signal between the wireless base station and a wireless fixed station for a domestic or company subscriber. A frequency bandwidth at, e.g., 26 GHz is used for the FWA system. A maximum transmission rate is 156 Mbit/sec in case of point-to-point (P-P) communication, and 10 Mbit/sec in case of point-to-multipoint (P-MP) communication.
FIG. 9A illustrates an FWA system that adopts a P-P approach. The FWA system shown in FIG. 9A performs wireless communications between one wireless base station 51 and one wireless subscriber station 52.
FIG. 9B depicts an FWA system that adopts a P-MP approach. The FWA system shown in FIG. 9B performs wireless communications between one wireless base station 61 and a plurality of wireless subscriber stations 62, 63, 64 and so on.
Each of the wireless subscriber stations 52, 62, 63, 64 can be a fixed station that is fixedly installed in a house or a business building.
For example, an FWA system, the so-called STEP1 system providing an analog telephone service, performs wireless communications between one WLL (Wireless Local Loop) call station (WCS) and WLL subscriber units (WSU) and connects the WCS to WLL access controllers (WACs) through optical fibers.
Another FWA system, the so-called STEP2 system, further provides an ISDN (Integrated Services Digital Network) service or an exclusive line service. The STEP2 system with multi-switching interfaces further includes a wireless entrance station, a wireless subscriber station for being installed in a house, or a WLL repeater station (WRS) between the WCS and the WAC.
As described above, the FWA system, instead of using optical fibers, uses an antenna of a wireless base station installed on the rooftop of a building in order to send a signal from the antenna to a wireless fixed station of a subscriber. In this way, an economic communication service can be provided even in a highly populated building area, a remote island, or a mountainous area in a short span of time.
In the FWA systems, however, communications are carried out by using a preset constant transmission power determined by considering certain unfavorable channel conditions such as rainfall attenuation and so on, which may results in the waste of valuable frequency resources available in fine weather conditions.
Further, the transceiver, which adopts a conventional adaptive modulation method, cannot be employed as it is to a communications system that performs communications by using asymmetric uplink and downlink transmission powers since the transceiver determines a modulation method according to the principle of reversibility in uplink and downlink only without considering the variation of the transmission powers.